High-power narrow-band interferences can disrupt broadband data transmission systems, navigation systems, high definition television and so on. Various conventional methods are available to reject interferences, such as:
U.S. Pat. No. 6,219,088, entitled “NTSC interference rejection filter”, describes an electronic, programmable filter that selectively removes interference, noise or distortion components from a baseband spectrum which is combined in a complex mixer with a synthesized frequency signal that shifts the spectrum of characteristic amount into the frequency domain such that the position of the interference component will be in the region about DC. Once shifted, the frequency components about DC are removed by a DC canceler circuit and the resulting spectrum is mixed with a subsequent synthesized frequency signal which shifts the spectrum back to its original representation and baseband. The frequency signals are developed by a programmable frequency synthesizer which may be programmed by a user together with an intelligence signal that defines the frequency location of an interference signal within the spectrum.
U.S. Pat. No. 5,263,048, entitled “Narrow band interference frequency excision method and means,” discloses a method for excising narrow band interferers in a spread spectrum signal communication, which includes the steps of: receiving the spread spectrum signal; digitizing the spread spectrum signal; transforming the digitized signal from the time domain to the frequency domain; discarding the magnitude of the digitized signal and replacing it with a normalized value; and transforming the digitized signal back to the time domain.
EP 1578022, entitled “Narrow band interference suppressor” discloses a plurality of interference signal elimination (ISE) circuits that are connected in series. Each ISE circuit includes a mixer for synthesizing a local oscillation signal of a variable frequency oscillator to eliminate an interference signal with a band elimination filter.
U.S. Pat. No. 6,219,376, entitled “Apparatuses and methods of suppressing a narrow-band interference with a compensator and adjustment loops”, discloses a compensator rejecting narrow-band interferences by means of adjustment loops. Two general methods of building such loops are considered. The first method is based on filtering the in-phase and quadrature components of the error vector—a difference of the interference vector and compensating vector. The second method is based on filtration of amplitude and full phase of the interference signal. Automatic tuning of mean frequency and effective interference band in the compensator is ensured.
U.S. Pat. No. 4,613,978, entitled “Narrowband interference suppression system”, discloses how strong narrowband signals interfering with the reception of a desired broadband signal are continuously suppressed by converting the received signal to a frequency-domain representation thereof, where strong narrowband interference components appear as strong impulse components. These impulsive components are blanked or clipped at a level that is a function of the average magnitude of the input signal. Resulting suppressed frequency-domain signals are reconverted to time-domain signals that are then available for further processing by conventional broadband signal receivers.
U.S. Pat. No. 8,345,808, entitled “Methods and apparatus for narrow band interference detection and suppression in ultra-wideband systems”, discloses a method to accurately estimate the center frequency of narrow-band interference (NBI). The exemplary method uses multi-stage autocorrelation-function (ACF) to estimate NBI frequency. The exemplary method estimates the frequency in multiple stages. Each stage performs an ACF operation on the received signals. The first stage gives an initial estimation and the following stages refine the estimation. The results of all stages are combined to produce the final estimation.
U.S. Pat. No. 6,975,673, entitled “Narrow-band interference rejecting spread spectrum radio system and method”, discloses a receiver and a method of rejection of narrow-band jamming signals using digital signal processing frequency domain techniques. The method includes transforming the received signal to a frequency domain signal and identifying narrow-band interference components in the frequency domain signal; suppressing the identified narrow-band interference components from the frequency domain signal.
However, what is needed is a simple digital adjustable filter for narrowband interference rejection that can be implemented without multiplications by shift-add operations.